1. Time - to build up their armies as both expected war eventually between them
2. Poland - the two divided Poland, Russia also invaded Lithunia Latvia but failed versus Finland
3. Resources - for Germany, it received raw materials from Russia that was used in its invasion of western europe
4. eventual Allied concessions made to Soviet Russia at end of WW2 - a Soviet Nazi alliance was simply too much for Britain to fight, even if US had joined the war earlier. When Germany attacked Russia, Britain itself was fighting for its life, so even as Russia must have welcomed allied aid in forms of military aid and equipment, the allied powers were the ones pursuing Russia to become a full pledged allied power ( it was earlier a Nazi ally or even an axis power itself ) Soviet Russia got the better deal in this alliance as eastern europe was conceded to Russia including division of Germany, UK and US were happy just to defeat the Nazis and they considered the huge loss of soviet lives ( instead of UK US lives ) was adequate price for the concessions. in short, by this secret agreement, but secret only in name, russia increased its asking price to join the allies.
The Romans, as well as the middle eastern, such as the Ottomans and the Turks.
